Olefins, particularly light olefins, have been traditionally produced from petroleum feedstocks by either catalytic or steam cracking. Oxygenates, however, are becoming an alternative feedstock for making light olefins, particularly ethylene and propylene. Promising oxygenate feedstocks are alcohols, such as methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates can be produced from a variety of sources including natural gas. Because of the relatively low-cost of these sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical source for light olefin production.
One way of producing olefins is by the conversion of methanol to olefins (MTO) catalyzed by a molecular sieve. Useful molecular sieves for converting methanol to olefin(s) are non-zeolitic molecular sieves, in particular metalloaluminophosphates such as the silicoaluminophosphates (SAPO's). For example, U.S. Pat. No. 4,499,327 to Kaiser, fully incorporated herein by reference, discloses making olefins from methanol using a variety of SAPO molecular sieve catalysts. The process can be carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1. Crystalline aluminosilicate zeolites have also been reported as catalysts for converting methanol and/or dimethyl ether to olefin-containing hydrocarbon mixtures. For example, U.S. Pat. No. 3,911,041 discloses that methanol can be converted to C2-C4 olefins by contacting the methanol at a temperature of 300° C. to 700° C. with a crystalline aluminosilicate zeolite catalyst which has a Constraint Index of 1-12, such as ZSM-5, and which contains at least 0.78% by weight of phosphorus incorporated in the crystal structure of the zeolite.
Zeolitic aluminosilicate molecular sieves contain a three-dimensional microporous crystal framework structure of [SiO2] and [AlO2] corner sharing tetrahedral units. Metalloaluminophosphate (MeAPO) molecular sieves, often qualified as non-zeolitic molecular sieves, contain a three-dimensional microporous crystal framework structure of [MO2], [AlO2] and [PO2] corner sharing tetrahedral units. When M is silicon, the molecular sieves are referred to as silicoaluminophosphate (SAPO) molecular sieves. There are a wide variety of aluminosilicate and MeAPO molecular sieves known in the art. Of these the more important examples as catalysts for the conversion of oxygenates to olefins include ZSM-5, ZK-5, ZSM-11, ZSM-12, ZSM-34, ZSM-35, erionite, chabazite, offretite, silicalite and other similar materials, SAPO-5, SAPO-11, SAPO-17, SAPO-18, SAPO-34, SAPO-35, SAPO-41, SAPO-56 and other similar materials. SAPO molecular sieves having the CHA framework type and especially SAPO-34 are particularly important catalysts. Another important class of SAPO molecular sieves consists of mixed or intergrown phases of molecular sieves having the CHA and AEI framework types. Examples of such materials are disclosed in WO 98/15496, published 16 Apr. 1998, and in WO 02/070407, published Sep. 12, 2002, both herein fully incorporated by reference.
While the aforementioned molecular sieves exhibit good catalytic properties in the conversion of methanol to olefins, there remains a need to improve their catalytic performance in order to decrease their selectivity to undesired saturated hydrocarbons and to increase their selectivity to desired light olefins (ethylene and propylene).
Various methods have been reported for treating and/or modifying crystalline molecular sieves in order to improve their catalytic performances. U.S. Pat. No. 5,250,484 discloses a method for making a surface inactivated catalyst composition comprising acidic porous crystalline material, in particular ZSM-23, having active internal Broensted acid sites and containing surface inactivating material having boron to nitrogen bonds. The method involves contacting the surface of the molecular sieve with aqueous ammonia borane solution. The modified catalysts are described for use in olefin oligomerization processes.
U.S. Pat. No. 6,046,371 discloses silylated silicoaluminophosphate compositions prepared by contacting calcined SAPOs with a silylating agent, preferably tetraalkyl orthosilicates and poly(alkylaryl)siloxanes. The silylated silicoaluminophosphate compositions are described as giving increased light olefin yields and decreased coke production, when used as catalysts in the conversion of oxygenated hydrocarbons to olefins.
U.S. Pat. No. 6,472,569 discloses catalyst systems comprising a silicoaluminophosphate impregnated with a compound selected from the group consisting of phosphoric acid, boric acid, tributyltin acetate, and combinations of any two or more thereof. These catalyst systems are described as giving increased light olefin yields and decreased coke production, when used as catalysts in the conversion of oxygenated hydrocarbons and/or ethers.
PCT Application WO 02/085514-A2 discloses a process for modifying a microporous framework defined by nanocages, such as SAPO-18 or SAPO-34. The modified microporous framework comprises and an inorganic compound in at least one of the nanocages, wherein said inorganic compound is a product formed by a reaction of a second inorganic molecule that has a kinetic diameter smaller than the kinetic diameter of the inorganic compound. The second inorganic compound is selected from the group consisting of PH3, SiH4, Si2H6 and B2H6. The inorganic compound may be selected from the group consisting of phosphoric acid, boric acid, silica, a product of the hydrolysis of PH3, a product of the hydrolysis of SiH4, a product of the hydrolysis of Si2H6, a product of the hydrolysis of B2H6, a product of the oxidation of PH3, a product of the oxidation of SiH4, a product of the oxidation of Si2H6 and a product of the oxidation of B2H6. This document discloses more specifically a process for modifying H-SAPO-34 by treating H-SAPO-34 with PH3 and methanol in a reactor at 250° C., followed by heating to 600° C. The method requires the presence of methanol to form P(CH3)3 and P(CH3)4+ species in the SAPO-34 nanocages. According to this document, the modified H-SAPO-34 delivers higher ethylene selectivity than unmodified H-SAPO-34.
Despite the various molecular sieve modifications reported in the literature, there remains a need to find other methods for improving molecular sieve catalytic performances, in order to decrease the selectivity of these molecular sieves to undesired saturated hydrocarbons and to increase their selectivity to desired light olefins (ethylene and propylene), when used as catalysts in the conversion of oxygenated hydrocarbons.